U.S. patent number 8,362,580 [Application Number 12/632,952] was granted by the patent office on 2013-01-29 for spin-transfer switching magnetic element utilizing a composite free layer comprising a superparamagnetic layer.
This patent grant is currently assigned to QUALCOMM Incorporated. The grantee listed for this patent is Wei-Chuan Chen, Seung H. Kang. Invention is credited to Wei-Chuan Chen, Seung H. Kang.
United States Patent |
8,362,580 |
Chen , et al. |
January 29, 2013 |
Spin-transfer switching magnetic element utilizing a composite free
layer comprising a superparamagnetic layer
Abstract
A system and method for forming a magnetic tunnel junction (MTJ)
storage element utilizes a composite free layer structure. The MTJ
element includes a stack comprising a pinned layer, a barrier
layer, and a composite free layer. The composite free layer
includes a first free layer, a superparamagnetic layer and a
nonmagnetic spacer layer interspersed between the first free layer
and the superparamagnetic layer. A thickness of the spacer layer
controls a manner of magnetic coupling between the first free layer
and the superparamagnetic layer.
Inventors: |
Chen; Wei-Chuan (San Diego,
CA), Kang; Seung H. (San Diego, CA) |
Applicant: |
Name |
City |
State |
Country |
Type |
Chen; Wei-Chuan
Kang; Seung H. |
San Diego
San Diego |
CA
CA |
US
US |
|
|
Assignee: |
QUALCOMM Incorporated (San
Diego, CA)
|
Family
ID: |
43618738 |
Appl.
No.: |
12/632,952 |
Filed: |
December 8, 2009 |
Prior Publication Data
|
|
|
|
Document
Identifier |
Publication Date |
|
US 20110133298 A1 |
Jun 9, 2011 |
|
Current U.S.
Class: |
257/421;
257/422 |
Current CPC
Class: |
H01L
43/08 (20130101); G11C 11/161 (20130101); H01L
27/228 (20130101); H01L 43/12 (20130101) |
Current International
Class: |
H01L
43/00 (20060101); H01L 43/12 (20060101) |
Field of
Search: |
;257/295,421-422,E43.001,E43.006 |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
Lee Kangho et al: "Effect of interlayer coupling in CoFeB/Ta/NiFe
free layers on the critical switching current of MgO-based magnetic
tunnel junctions", Journal of Applied Physics, American Institute
of Physics. New York, US, vol. 106, No. 2, Jul. 29, 2009, pp.
24513-24513, XP012123656, ISSN: 0021-8979, DOI:10.1063/1.3184423.
cited by applicant .
Written Opinion, International Search Report--PCT/US2010/059527,
International Search Authority--European Patent Office Mar. 16,
2011. cited by applicant.
|
Primary Examiner: Nguyen; Cuong Q
Attorney, Agent or Firm: Talpalatsky; Sam Pauley; Nicholas
J. Velasco; Jonathan T.
Claims
What is claimed is:
1. A magnetic tunnel junction (MTJ) storage element comprising: a
stack comprising a pinned layer and a barrier layer; and a
composite free layer formed on the barrier layer, comprising a
first free layer, a nonmagnetic spacer layer and a
superparamagnetic layer, such that the spacer layer is interspersed
between the first free layer and the superparamagnetic layer.
2. The MTJ storage element of claim 1, further comprising an
interlayer exchange coupling between the first free layer and the
superparamagnetic layer.
3. The MTJ storage element of claim 2, wherein a magnetic
polarization of the first free layer is aligned parallel to a
magnetic polarization of the superparamagnetic layer.
4. The MTJ storage element of claim 2, wherein a magnetic
polarization of the first free layer is aligned anti-parallel to a
magnetic polarization of the superparamagnetic layer.
5. The MTJ storage element of claim 1, further comprising an
interlayer fringe coupling between the first free layer and the
superparamagnetic layer.
6. The MTJ storage element of claim 5, wherein a magnetic
polarization of the first free layer is aligned anti-parallel to a
magnetic polarization of the superparamagnetic layer.
7. The MTJ storage element of claim 1, wherein the
superparamagnetic layer is formed from a ferromagnetic layer of
reduced thickness.
8. The MTJ storage element of claim 1, wherein the
superparamagnetic layer is formed from an antiferromagnetic layer
of reduced thickness.
9. The MTJ storage element of claim 1, wherein the
superparamagnetic layer is formed from a nonmagnetic material doped
with ferromagnetic elements.
10. The MTJ storage element of claim 1, wherein the
superparamagnetic layer is formed from a ferromagnetic material
doped with nonmagnetic elements.
11. The MTJ storage element of claim 1, wherein the
superparamagnetic layer is formed from a laminated structure
comprising one or more layers of ferromagnetic elements,
interspersed with one or more layers of nonmagnetic elements.
12. The MTJ storage element of claim 1, further comprising an
antiferromagnetic material in contact with the pinned layer, formed
below the pinned layer.
13. The MTJ storage element of claim 1, wherein the pinned layer is
a pinned layer stack comprising two or more layers.
14. The MTJ storage element according to claim 1, wherein the
storage element is applied in an electronic device, selected from
the group consisting of a set top box, music player, video player,
entertainment unit, navigation device, communications device,
personal digital assistant (PDA), fixed location data unit, and a
computer, into which the MTJ storage element is integrated.
15. The MTJ storage element according to claim 1, wherein the
storage element is integrated in a Spin Transfer Torque
Magnetoresistive Random Access Memory (STT-MRAM).
16. The STT-MRAM device according to claim 15, wherein the STT-MRAM
device is integrated in at least one semiconductor die.
17. A method of forming a magnetic tunnel junction (MTJ) storage
element, the method comprising: forming a stack comprising a pinned
layer and a barrier layer; and forming a composite free layer on
top of the barrier layer comprising a first free layer, a
nonmagnetic spacer layer and a superparamagnetic layer, such that
the spacer layer is interspersed between the first free layer and
the superparamagnetic layer.
18. The method of claim 17, further comprising coupling the first
free layer and the superparamagnetic layer via interlayer exchange
coupling.
19. The method of claim 18, wherein a magnetic polarization of the
first free layer is aligned parallel to a magnetic polarization of
the superparamagnetic layer.
20. The method of claim 18, wherein a magnetic polarization of the
first free layer is aligned anti-parallel to a magnetic
polarization of the superparamagnetic layer.
21. The method of claim 17, further comprising: coupling the first
free layer the superparamagnetic layer via interlayer fringe
coupling.
22. The method of claim 21, wherein a magnetic polarization of the
first free layer is aligned anti-parallel to a magnetic
polarization of the superparamagnetic layer.
23. The method of claim 17, wherein the superparamagnetic layer is
formed by reducing the thickness of a ferromagnetic or
antiferromagnetic layer.
24. The method of claim 17, wherein the superparamagnetic layer is
formed by doping a nonmagnetic material with ferromagnetic
elements.
25. The method of claim 17, wherein the superparamagnetic layer is
formed by doping a ferromagnetic material with nonmagnetic
elements.
26. The method of claim 17, wherein the superparamagnetic layer is
formed by interspersing one or more layers of nonmagnetic elements
with one or more layers of ferromagnetic elements.
27. The method according to claim 17, wherein the MTJ storage
element is applied in an electronic device, selected from the group
consisting of a set top box, music player, video player,
entertainment unit, navigation device, communications device,
personal digital assistant (PDA), fixed location data unit, and a
computer, into which the MTJ storage element is integrated.
28. The method according to claim 17, wherein the MTJ storage
element is integrated in a Spin Transfer Torque Magnetoresistive
Random Access Memory (STT-MRAM).
29. A magnetic tunnel junction (MTJ) storage element comprising: a
first magnetic means for holding a first polarization; a composite
magnetic means for holding a second polarization comprising
ferromagnetic means; superparamagnetic means; and nonmagnetic means
interspersed between the ferromagnetic means and the
superparamagnetic means, wherein a thickness of the nonmagnetic
means controls a manner of coupling between the ferromagnetic means
and the superparamagnetic means; and insulating means interspersed
between the first magnetic means and composite magnetic means to
enable a flow of tunneling current between the first magnetic means
and the composite magnetic means.
30. The MTJ storage element of claim 29, wherein the manner of
coupling between the ferromagnetic means and the superparamagnetic
means is interlayer exchange coupling.
31. The MTJ storage element of claim 30, wherein a magnetic
polarization of the ferromagnetic means is aligned parallel to a
magnetic polarization of the superparamagnetic means.
32. The MTJ storage element of claim 30, wherein a magnetic
polarization of the ferromagnetic means is aligned anti-parallel to
a magnetic polarization of the superparamagnetic means.
33. The MTJ storage element of claim 29, wherein the manner of
coupling between the ferromagnetic means and the superparamagnetic
means is interlayer fringe coupling.
34. The MTJ storage element of claim 33, wherein a magnetic
polarization of the ferromagnetic means is aligned anti-parallel to
a magnetic polarization of the superparamagnetic means.
35. The MTJ storage element of claim 29, wherein the
superparamagnetic means is formed from a ferromagnetic or
anti-ferromagnetic material of reduced thickness.
36. The MTJ storage element of claim 29, wherein the
superparamagnetic means is formed from a nonmagnetic material doped
with ferromagnetic elements.
37. The MTJ storage element of claim 29, wherein the
superparamagnetic means is formed from a ferromagnetic material
doped with nonmagnetic elements.
38. The MTJ storage element of claim 29, wherein the
superparamagnetic means is formed from a laminated structure
comprising one or more layers of ferromagnetic elements,
interspersed with one or more layers of nonmagnetic elements.
39. The MTJ storage element according to claim 29, wherein the MTJ
storage element is applied in an electronic device, selected from
the group consisting of a set top box, music player, video player,
entertainment unit, navigation device, communications device,
personal digital assistant (PDA), fixed location data unit, and a
computer, into which the MTJ storage element is integrated.
40. The MTJ storage element according to claim 29, wherein the MTJ
storage element is integrated in a Spin Transfer Torque
Magnetoresistive Random Access Memory (STT-MRAM).
41. A method of forming a magnetic tunnel junction (MTJ) storage
element, the method comprising: step for forming a stack comprising
a pinned layer and a barrier layer; and step for forming a
composite free layer on top of the barrier layer comprising a first
free layer, a nonmagnetic spacer layer and a superparamagnetic
layer, such that the spacer layer is interspersed between the first
free layer and the superparamagnetic layer.
42. The method of claim 41, further comprising coupling the first
free layer and the superparamagnetic layer via interlayer exchange
coupling.
43. The method of claim 42, wherein a magnetic polarization of the
first free layer is aligned parallel to a magnetic polarization of
the superparamagnetic layer.
44. The method of claim 42, wherein a magnetic polarization of the
first free layer is aligned anti-parallel to a magnetic
polarization of the superparamagnetic layer.
45. The method of claim 41, further comprising coupling the first
free layer the superparamagnetic layer via interlayer fringe
coupling.
46. The method of claim 45, wherein a magnetic polarization of the
first free layer is aligned anti-parallel to a magnetic
polarization of the superparamagnetic layer.
47. The method of claim 41, wherein the superparamagnetic layer is
formed by reducing the thickness of a ferromagnetic or
antiferromagnetic layer.
48. The method of claim 41, wherein the superparamagnetic layer is
formed by doping a nonmagnetic material with ferromagnetic
elements.
49. The method of claim 41, wherein the superparamagnetic layer is
formed by doping a ferromagnetic material with nonmagnetic
elements.
50. The method of claim 41, wherein the superparamagnetic layer is
formed by interspersing one or more layers of nonmagnetic elements
with one or more layers of ferromagnetic elements.
51. The method according to claim 41, wherein the MTJ storage
element is applied in an electronic device, selected from the group
consisting of a set top box, music player, video player,
entertainment unit, navigation device, communications device,
personal digital assistant (PDA), fixed location data unit, and a
computer, into which the MTJ storage element is integrated.
52. The method according to claim 41, wherein the MTJ storage
element is integrated in a Spin Transfer Torque Magnetoresistive
Random Access Memory (STT-MRAM).
Description
FIELD OF DISCLOSURE
Disclosed embodiments are related to employing a composite free
layer comprising a superparamagnetic layer in a Magnetic Tunnel
Junction (MTJ) storage element usable in a Spin Transfer Torque
Magnetoresistive Random Access Memory (STT-MRAM) cell.
BACKGROUND
Magnetoresistive Random Access Memory (MRAM) is a non-volatile
memory technology that uses magnetic elements. For example, Spin
Transfer Torque Magnetoresistive Random Access Memory (STT-MRAM)
uses electrons that become spin-polarized as the electrons pass
through a thin film (spin filter). STT-MRAM is also known as Spin
Transfer Torque RAM (STT-RAM), Spin Torque Transfer Magnetization
Switching RAM (Spin-RAM), and Spin Momentum Transfer (SMT-RAM).
FIG. 1 illustrates a conventional STT-MRAM bit cell 100. The
STT-MRAM bit cell 100 includes magnetic tunnel junction (MTJ)
storage element 105, a transistor 101, a bit line 102 and a word
line 103. The MTJ storage element is formed, for example, from at
least two ferromagnetic layers (a pinned layer and a free layer),
each of which can hold a magnetic field or polarization, separated
by a thin non-magnetic insulating layer (tunneling barrier).
Electrons from the two ferromagnetic layers can penetrate through
the tunneling barrier due to a tunneling effect under a bias
voltage applied to the ferromagnetic layers. The magnetic
polarization of the free layer can be reversed so that the polarity
of the pinned layer and the free layer are either substantially
aligned (parallel) or opposite (anti-parallel). The resistance of
the electrical path through the MTJ will vary depending on the
alignment of the polarizations of the pinned and free layers. This
variance in resistance can be used to program and read the bit cell
100. The STT-MRAM bit cell 100 also includes a source line 104, a
sense amplifier 108, read/write circuitry 106 and a bit line
reference 107. Those skilled in the art will appreciate the
operation and construction of the memory cell 100.
For example, the bit cell 100 may be programmed such that a binary
value "1" is associated with an operational state wherein the
polarity of the free layer is parallel to the polarity of the
pinned layer. Correspondingly, a binary value "0" may be associated
with an anti-parallel orientation between the two ferromagnetic
layers. A binary value may thus be written to the bit cell by
changing the polarization of the free layer. A sufficient current
density (typically measured in Amperes/centimeter.sup.2) generated
by the electrons flowing across the tunneling barrier is required
to change the polarization of the free layer. The minimum current
density required to switch the polarization of the free layer is
also called switching current density. Decreasing the value of the
switching current density leads to beneficially lowering the power
consumption of the MTJ cells. Additionally, lower switching current
density enables smaller device dimensions and a correspondingly
higher density of MTJ cells in an STT-MRAM integrated circuit.
Existing techniques to reduce the switching current density may
adversely affect the thermal stability of the MTJ cell.
Accordingly, there is a need for decreasing the switching current
density without impacting the thermal stability of the device.
SUMMARY
Exemplary embodiments of the invention are directed to systems and
method for employing a composite free layer comprising a
superparamagnetic layer in a Magnetic Tunnel Junction (MTJ) storage
element usable in a Spin Transfer Torque Magnetoresistive Random
Access Memory (STT-MRAM) cell.
For example, an exemplary embodiment is directed to an MTJ storage
element comprising a stack comprising a pinned layer and a barrier
layer; and a composite free layer formed on the barrier layer,
comprising a first free layer a nonmagnetic spacer layer and a
superparamagnetic layer, such that the spacer layer is interspersed
between the first free layer and the superparamagnetic layer.
Another exemplary embodiment is directed to a method of forming an
MTJ storage element, the method comprising forming a stack
comprising a pinned layer and a barrier layer; and forming a
composite free layer on top of the barrier layer comprising a first
free layer, a nonmagnetic spacer layer and a superparamagnetic
layer, such that the spacer layer is interspersed between the first
free layer and the superparamagnetic layer.
Yet another exemplary embodiment is directed to an MTJ storage
element comprising a first magnetic means for holding a first
polarization; a composite magnetic means for holding a second
polarization comprising ferromagnetic means; superparamagnetic
means; and nonmagnetic means interspersed between the ferromagnetic
means and the superparamagnetic means, wherein a thickness of the
nonmagnetic means controls a manner of coupling between the
ferromagnetic means and the superparamagnetic means; and insulating
means interspersed between the first magnetic means and composite
magnetic means to enable a flow of tunneling current between the
first magnetic means and the composite magnetic means.
Another exemplary embodiment is directed to a method of forming an
MTJ storage element, the method comprising step for forming a stack
comprising a pinned layer and a barrier layer; and step for forming
a composite free layer on top of the barrier layer comprising a
first free layer, a nonmagnetic spacer layer and a
superparamagnetic layer, such that the spacer layer is interspersed
between the first free layer and the superparamagnetic layer.
BRIEF DESCRIPTION OF THE DRAWINGS
The accompanying drawings are presented to aid in the description
of embodiments of the invention and are provided solely for
illustration of the embodiments and not limitation thereof.
FIG. 1 illustrates a conventional Spin Transfer Torque
Magnetoresistive Random Access Memory (STT-MRAM) cell array.
FIG. 2 illustrates an exemplary MTJ storage element utilizing a
composite free layer structure comprising a superparamagnetic
layer.
FIG. 3 illustrates different coupling effects in exemplary
embodiments.
FIG. 4 illustrates interlayer fringe coupling effects in exemplary
embodiments.
FIG. 5 illustrates interlayer exchange coupling effects in
exemplary embodiments
FIG. 6 illustrates different techniques to form an exemplary
superparamagnetic layer.
FIG. 7 illustrates a flowchart for forming a memory device.
DETAILED DESCRIPTION
Aspects of the invention are disclosed in the following description
and related drawings directed to specific embodiments of the
invention. Alternate embodiments may be devised without departing
from the scope of the invention. Additionally, well-known elements
of the invention will not be described in detail or will be omitted
so as not to obscure the relevant details of the invention.
The word "exemplary" is used herein to mean "serving as an example,
instance, or illustration." Any embodiment described herein as
"exemplary" is not necessarily to be construed as preferred or
advantageous over other embodiments. Likewise, the term
"embodiments of the invention" does not require that all
embodiments of the invention include the discussed feature,
advantage or mode of operation. The terminology used herein is for
the purpose of describing particular embodiments only and is not
intended to be limiting of embodiments of the invention.
As used herein, the singular forms "a", "an" and "the" are intended
to include the plural forms as well, unless the context clearly
indicates otherwise. It will be further understood that the terms
"comprises", "comprising,", "includes" and/or "including", when
used herein, specify the presence of stated features, integers,
steps, operations, elements, and/or components, but do not preclude
the presence or addition of one or more other features, integers,
steps, operations, elements, components, and/or groups thereof.
The disclosed embodiments recognize that, with conventional
methods, it may be difficult to decrease the switching current
density of MTJ devices while maintaining their thermal stability.
The physical characteristics of the ferromagnetic used in MTJ cells
include a large internal magnetic field at room temperature.
Reversing the polarization of ferromagnetic layers requires a
relatively large current density, unless accompanied by factors
such as an increased thermal energy.
Existing techniques to reduce the switching current density include
the use of "spin diffusion layers", as in Huai et al.,
"Current-Switched Spin-Transfer Magnetic Devices with Reduced
Spin-Transfer Switching Current Density", United States Patent
Application Publication, Pub. No. US 2007/0171694 A1. The spin
diffusion layers diffuse the electron spins outside the MTJ. As a
result, the spin dependent current flowing through the MTJ may be
diminished in the layers outside the free layer so that most of the
spin dependent current may be confined in the magnetically active
part of the MTJ stack. This may lead to a reduction of the
switching current density.
Prior art techniques also include the use of low saturation
magnetization materials for forming the free layer. For example,
Nguyen et al., "Spin Transfer Magnetic Element Having Low
Saturation Magnetization Free Layers", United States Patent
Application Publication, Pub. No. US 2007/0159734 A1, which is
incorporated in its entirety herein, describes techniques wherein
the free layer includes ferromagnetic materials diluted with
nonmagnetic materials and/or ferrimagnetically doped to provide low
saturation magnetizations. Lowering the saturation magnetization of
the free layer may reduce the switching current density.
Exemplary embodiments recognize that in contrast to ferromagnetic
materials, the magnetization of superparamagnetic materials is
significantly low at room temperature. Accordingly, a very low
current density is required to reverse the polarization of a
composite free layer with a superparamagnetic material at room
temperature. While existing techniques include the limitations of
free layers formed of ferromagnetic materials, disclosed
embodiments provide techniques wherein the free layer may
advantageously include superparamagnetic materials. Exemplary
embodiments detail the use of superparamagnetic materials in
lowering the current density while enhancing the thermal stability
of the MTJ.
FIG. 2 illustrates the MTJ cell 105 according to an exemplary
embodiment. An antiferromagnetic (AFM) layer 204 is first formed on
a bottom electrode 202, and then a first ferromagnetic layer is
formed on top of the AFM layer. The first ferromagnetic layer is
"pinned" with a fixed magnetic polarization to form a pinned layer.
The pinned layer may include one or more layers, such as a bottom
pinned layer 206, a coupling layer 208 typically formed of a
non-magnetic metal such as ruthenium, and a top pinned layer 210.
Pinned layers 206, 208 and 210 may be collectively referred to as a
pinned layer stack. A tunneling barrier layer 212 is formed of an
insulator such as a metal oxide on top of the pinned layer. A free
layer with variable magnetic polarization is formed on top of the
barrier layer. The free layer may include a first free layer 214, a
non-magnetic spacer layer 216 and a superparamagnetic layer 218 as
shown in FIG. 2. Such a multilayered free layer structure is called
a composite free layer or "synthetic" free layer. It will be
appreciated that the formation of MTJ devices with synthetic free
layers is well known. A top electrode (not shown) is formed on top
of the free layer.
The electrons tunneling through the barrier layer 212 from the
pinned layers enter the first free layer 214, causing an effect on
the magnetic polarization of the first free layer 214. If switching
current density is achieved, depending on the direction of spin of
the majority of electrons tunneling through the barrier 212, the
magnetic polarization of the first free layer may become
substantially aligned (parallel) to the magnetic polarization of
the pinned layers, or substantially aligned opposite
(anti-parallel) to the magnetic polarization of the pinned layers.
The directional arrows illustrated within the layers are merely an
illustrative aid to depict an exemplary direction of polarization
of the layer, and the embodiments are not limited in any manner by
these illustrations. The spacer layer 216 is non-magnetic. The
magnetization of the superparamagnetic layer 218 may be influenced
by one of three methods.
The first method of coupling the superparamagnetic layer 218 is
illustrated in FIG. 3A. If the nonmagnetic spacer layer 216 is made
sufficiently thin, the superparamagnetic layer 218 may become
magnetically "coupled" to the first free layer 214. The magnetic
polarization of the superparamagnetic layer 218 is derived from,
and aligned with, the polarization of the first free layer 214 due
to an exchange of energy between the two layers. This manner of
coupling is called "direct interlayer coupling". The coupling
strength is controlled by factors such as the thickness of the
spacer layer 216.
The spacer layer 216 may be formed from a nonmagnetic material such
as Ru, which leads to a second coupling mechanism known as "RKKY
coupling" or "indirect interlayer coupling". In this manner of
coupling, the thickness of the nonmagnetic material determines
whether the coupling between the first free layer 214 and
superparamagnetic layer 218 is parallel or anti-parallel.
FIG. 3B illustrates a third method of coupling between the free
layers 214 and 218. If the thickness of the spacer layer 216 is
increased, the interlayer coupling effect is diminished. However,
the effect of fringe fields between the sidewalls of the two free
layers 214 and 218 may lead to a magnetic coupling between them.
This manner of coupling is usually referred to as "interlayer
fringe coupling". The polarization of the superparamagnetic layer
218 is usually aligned anti-parallel to the polarization of the
first free layer 214 under the effect of fringe coupling.
In the case of interlayer exchange coupling (first and second
methods as described above, as shown in FIG. 3A), it is possible to
achieve a strong coupling between free layers 214 and 218. The
coupling strength may depend on factors which include the thickness
and material of the spacer layer 216. If the spacer layer 216 is
sufficiently thin, a strong coupling effect may be formed. Under a
strongly coupled synthetic free layer structure, the two free
layers 214 and 218 may behave as though they were one single free
layer. This facilitates a "coherent" switching of the free layer.
i.e., when a switching current causes the first free layer 214 to
switch polarization, the coupling effect causes an instantaneous
switching effect on the superparamagnetic layer 218.
The maximum polarization which can be induced in a magnetic
material is called saturation magnetization. It will be understood
that achieving a lower saturation magnetization will lead to lower
switching current density. In the composite free layer structure
with strong exchange coupling as explained above, a sufficient
current density is only required to switch the first free layer
214, but the net effect is equivalent to switching both free layers
214 and 218. Moreover, a composite free layer can have a lower
switching current density than a single free layer, such as 214. As
described previously, existing composite free layer structures are
formed from ferromagnetic materials. Hence the advantages of using
a composite free layer, to achieve a lower switching current
density, are limited by the inherent magnetic properties of
ferromagnetic materials. However, according to an exemplary
embodiment, utilizing a superparamagnetic layer 218 in a strongly
coupled synthetic free layer, leads to a significantly lower
switching current density.
Superparamagnetic materials are composed of small ferromagnetic
clusters. But these clusters are of such small dimensions that
their polarizations may flip randomly under thermal fluctuations.
As a result, the net polarization of a superparamagnetic material
averages out to zero in the absence of an external magnetic field.
However, when an external magnetic field is applied, the
superparamagnetic material becomes easily polarized, even at room
temperature. On the other hand, ferromagnetic materials have an
inherent non-zero polarization at room temperatures. Accordingly,
reversing the polarization of ferromagnetic materials at room
temperature needs a significantly greater magnetic energy, than the
polarization of a superparamagnetic material.
Using a superparamagnetic material to form the superparamagnetic
layer 218 in the case of strongly coupled free layers which exhibit
coherent switching has various beneficial effects. For example, the
coupling fields required to polarize the superparamagnetic free
layer 218 are lower, which in turn leads to a stable domain
structure resulting in the better uniformity of switching behavior.
Exemplary embodiments use a superparamagnetic material to form the
superparamagnetic layer 218 in a strongly coupled composite free
layer structure, which results in lower saturation magnetization of
the composite free layer and an enhancement of the spin-torque
efficiency in the STT-MRAM.
Exemplary embodiments also include the use of superparamagnetic
materials in the case of composite free layers which exhibit fringe
coupling behavior. In contrast to the coherent switching
characteristics exhibited by exchange coupling, the coupling effect
in fringe coupling is weaker, and the switching behavior is more
stochastic. Sometimes the fringe coupling may be so weak that it
may be the equivalent of no magnetic coupling at all. In such
scenarios, the composite free layer exhibits a "non-coherent"
switching behavior, i.e., the first free layer 214 undergoes
switching at a first point in time. Due to the extremely weak
coupling behavior, the superparamagnetic layer 218 is caused to
switch subsequently at a second point in time. It will be
appreciated that such non-coherent switching leads to an increased
spin-torque efficiency, and hence a reduced switching current
density.
FIG. 4 illustrates exemplary embodiments using a superparamagnetic
layer 402. FIG. 4A illustrates randomly aligned magnetic clusters
in the superparamagnetic layer 402 taken in isolation. In the
absence of an external magnetic field, the polarizations of the
magnetic clusters cancel out, resulting in a net magnetic moment of
zero. Depending on the type of coupling behavior (i.e. exchange
coupling or fringe coupling), and the thickness and material of the
nonmagnetic spacer layer, the superparamagnetic layer 402 may be
polarized parallel or anti-parallel with respect to the first free
layer 214. The first free layer 214 may itself become polarized
parallel (P) or anti-parallel (AP) with respect to the top pinned
layer, depending on the spin direction of the majority of electrons
tunneling through the barrier layer 212.
FIG. 4B represents an exemplary embodiment wherein the first free
layer 214 is polarized anti-parallel (AP) with respect to the top
pinned layer 210. Fringe coupling effects in this embodiment cause
the superparamagnetic layer 402 to be polarized anti-parallel to
the first free layer 214. This embodiment is referred to an "AP
state" of MTJ 105. FIG. 4C represents a "P state" of MTJ 105.
When the magnetic moment between a superparamagnetic layer and a
first free layer is anti-parallel, the switching current density
required for "P to AP" polarizations of the superparamagnetic layer
402 is effectively the same as the switching current density
required for "AP to P" polarizations. The use of superparamagnetic
materials to form the superparamagnetic layer 402 can result in
uniform switching behavior by promoting a stable domain
structure.
FIG. 5 illustrates yet another exemplary embodiment wherein the
coupling mechanism between the two free layers 214 and 218 is
interlayer exchange coupling. As explained previously, the
superparamagnetic layer 402 may be aligned either parallel to the
first free layer 214 (as illustrated in FIGS. 5A-B) or
anti-parallel to the first free layer 214 (as illustrated in FIGS.
5C-D), based on the thickness and material of the nonmagnetic
spacer layer 216.
Both parallel and anti-parallel alignments between the first free
layer 214 and superparamagnetic layer 402 advantageously reduce the
switching current density of the MTJ cell by enhancing the
spin-torque efficiency and lowering the saturation magnetization of
the free layer. For the parallel alignment between first free layer
214 and superparamagnetic layer 402, the increase of the
spin-torque efficiency is due to the reflection of major spin
electron by superparamagnetic free layer during the switching from
AP to P. Spin current that comes from the barrier layer 212 enters
the first free layer 214, which acts as a spin filter, changing the
magnitude and direction of the spin current. This spin current is
reflected back at the interface between first free layer 214 and
superparamagnetic layer 402. The torque exerted by the reflected
current assists the torque of the spin current entering the free
layer 214, causing the free layer 214 to switch at a lower
switching current density. In other words, the enhancement of the
spin-torque efficiency is due to the phase difference between the
first free layer 214 and superparamagnetic layer 402. The
reference, Yen et al., "Reduction in critical current density for
spin torque transfer switching with composite free layer", Applied
Physics Letters 93, 092504 (2008), provides further details on the
relationship between phase difference in composite free layer
structures and the associated reflection of spin current,
contributing to lower switching current density.
For the anti-parallel alignment between first free layer 214 and
superparamagnetic layer 402, the increase of the spin-torque
efficiency is due to the enhancement (polarization) of minor spin
electron by superparamagnetic free layer during the switching from
P to AP. In the case of anti-parallel alignment as illustrated in
FIG. 5D, the minority electron spin direction contributes
significantly to balancing the switching current density between
the "P to AP" polarization embodiments.
Methods for forming the superparamagnetic layer 402 are well known
to one of ordinary skill in the art and will not be described in
detail herein. FIG. 6 illustrates a few conventional techniques for
forming a superparamagnetic layer 402. FIG. 6A demonstrates a
common technique wherein the thickness of a layer formed from a
ferromagnetic or antiferromagnetic material is reduced in
dimension. As the thickness is reduced, the ferromagnetic or
antiferromagnetic material starts losing its inherent magnetic
field, resulting in randomly aligned magnetic clusters. When the
thickness is reduced below a certain value (typically less than 15
A.degree. for a ferromagnetic material, and less than 50 A.degree.
for an antiferromagnetic material) the net magnetic moment of the
randomly polarized magnetic clusters becomes zero, and the material
is said to be superparamagnetic.
A nonmagnetic material doped with ferromagnetic materials as shown
in FIG. 6B may also give rise to a net magnetic moment of zero, but
under the influence of an external magnetic field, the
ferromagnetic clusters can be easily polarized to align with the
external field, at room temperature. A superparamagnetic layer may
be formed using the technique illustrated in FIG. 6B. FIG. 6C shows
a similar technique to form a superparamagnetic layer, by doping a
ferromagnetic layer with nonmagnetic materials. FIG. 6D represents
yet another method for forming a superparamagnetic layer, by using
a laminated ferromagnetic/nonmagnetic multilayer. The
superparamagnetic layer 402 in exemplary embodiments may be
advantageously formed using techniques which include the techniques
of FIGS. 6A-D.
Accordingly, disclosed embodiments with a composite free layer
comprising a superparamagnetic layer advantageously exhibit reduced
switching current density due to low saturation magnetization and
enhancement of spin efficiency. Further, as compared to composite
free layers which only include ferromagnetic materials, the
disclosed composite free layer structures with a superparamagnetic
layer contribute to enhanced thermal stability due to a stable
domain structure. Moreover the larger volume of the disclosed
composite free layer structures, as compared to conventional single
layer free layer structures, also improves the thermal stability of
the MTJ device.
It will be appreciated from the foregoing disclosure that
embodiments can include various methods including those used to
form the memory devices described herein. Accordingly, as
illustrated in FIG. 7, an embodiment can include a method for
forming a memory device having a magnetic tunnel junction (MTJ)
storage element. A stack is formed (step 702) having an
antiferromagnetic layer, a pinned layer stack and a barrier layer.
A composite free layer is formed on top of the barrier layer (step
704). The composite free layer includes a first free layer, a
nonmagnetic spacer layer and a superparamagnetic layer, such that
the spacer layer is interspersed between the first free layer and
the superparamagnetic layer. It will be appreciated that FIG. 7 and
the foregoing description are not intended to limit the embodiments
to the illustrated and expressly discussed feature. Embodiments can
further include any of the additional steps/functionalities
described herein.
It will be appreciated that memory devises including the MTJ
storage elements described herein may be included within a mobile
phone, portable computer, hand-held personal communication system
(PCS) unit, portable data units such as personal data assistants
(PDAs), GPS enabled devices, navigation devices, settop boxes,
music players, video players, entertainment units, fixed location
data units such as meter reading equipment, or any other device
that stores or retrieves data or computer instructions, or any
combination thereof. Accordingly, embodiments of the disclosure may
be suitably employed in any device which includes active integrated
circuitry including memory having MTJ storage elements as disclosed
herein.
The foregoing disclosed devices and methods can be designed and can
be configured into GDSII and GERBER computer files, stored on a
computer readable media. These files are in turn provided to
fabrication handlers who fabricate devices based on these files.
The resulting products are semiconductor wafers that are then cut
into semiconductor die and packaged into a semiconductor chip. The
chips are then employed in devices described above.
Accordingly, embodiments can include machine-readable media or
computer-readable media embodying instructions which when executed
by a processor transform the processor and any other cooperating
elements into a machine for performing the functionalities
described herein as provided for by the instructions.
While the foregoing disclosure shows illustrative embodiments, it
should be noted that various changes and modifications could be
made herein without departing from the scope of the invention as
defined by the appended claims. The functions, steps and/or actions
of the method claims in accordance with the embodiments described
herein need not be performed in any particular order. Furthermore,
although elements of the embodiments may be described or claimed in
the singular, the plural is contemplated unless limitation to the
singular is explicitly stated.
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